Recently, demand for wireless (or radio) data services is explosively increasing, and evolution from a radio voice service to a radio data service triggers the requirement of a gradual increase in a wireless capacity. Such demand prompts the radio service providers and wireless equipment manufacturers to seek an improvement of a data transmission rate of a wireless system, and motivates a great deal of research.
One of the systems considered a post-3rd generation system is an orthogonal frequency division multiplexing (OFDM) system that attenuates an inter-symbol interference with a low complexity. In the OFDM, serially input data are converted into N number of parallel data, which are included in N number of subcarriers so as to be transmitted. The subcarriers maintain orthogonality in a frequency domain. Orthogonal frequency division multiple access (OFDMA) refers to a multi-access method that accomplishes multiple accesses by independently providing portions of available subcarriers to each user in a system using OFDM as a modulation scheme.
One of the major problems of the OFDM/OFDMA system, however, is that a peak-to-average power ratio (PAPR) is possibly very high. The PAPR issue arises as a peak amplitude of a transmission signal appears to be very large compared with an average amplitude, which results from the fact that OFDM symbols overlap with N number of sinusoidal signals on mutually different subcarriers. PAPR is especially problematic for a terminal sensitive to power consumption in relation to a battery capacity. In order to reduce power consumption, the PAPR needs to be lowered.
A single carrier-frequency division multiple access (SC-FDMA) is one of systems proposed to lower the PAPR. The SC-FDMA is a scheme obtained by combining frequency division multiple access (FDMA) to single carrier-frequency division equalization (SC-FDE). The SC-FDMA, which is similar to the OFDMA in that data is modulated and demodulated in time and frequency domains by using discrete Fourier transform (DFT), though, ensures a low PAPR of a transmission signal, so it is advantageous for reducing transmission power. In particular, the SC-FDMA is advantageous for uplink communication from a terminal sensitive to transmission power in relation to the usage of a battery to a base station.
Meanwhile, when a transport channel undergoes deep fading, unless a different version or a replica of a transmitted signal is additionally transmitted, a receiver can hardly determine the transmitted signal. Resources corresponding to such different version or replica are called diversity which is one of the most significant factors contributing to reliable transmission over radio channels. The use of diversity can maximize a data transmission capacity or data transmission reliability, and a system implementing diversity with multiple transmission antennas and multiple reception antennas is called a multiple input multiple output (MIMO) system.
Schemes for implementing diversity in the MIMO system include a SFBC (Space Frequency Block Code), STBC (Space Time Block Code), CDD (Cyclic Delay Diversity), FSTD (frequency switched transmit diversity), TSTD (time switched transmit diversity), PVS (Precoding Vector Switching), Spatial Multiplexing (SM), and the like. The SM scheme includes a closed-loop spatial multiplexing scheme that performs precoding upon obtaining feedback information again from a receiver and an open-loop spatial multiplexing scheme that performs precoding without a feedback.
The OFDM system uses a spatial multiplexing scheme that performs precoding by using a unitary matrix including only non-zero elements. In case of performing precoding by using such unitary matrix, when modulation symbols are added to each other by the matrix, PAPR increases. In addition, for a spatial multiplexing gain, the OFDMA system uses a method in which signals combined through DFT after modulation are switched by a diagonal matrix having a linear phase rotation by resource elements so as to be mapped to frequency resources. However, application of such mapping to the SC-FDMA system cannot guarantee a low PAPR after performing IFFT.
The system employing the SC-FDMA modulation is sensitive to power consumption, so it is not proper to apply the spatial multiplexing scheme used for the OFDM system to the system employing the SC-FDMA modulation. Thus, a spatial multiplexing scheme that does not increase a PAPR in the SC-FDMA system should be considered.